1. Field of the Invention
This invention relates to secondary electrochemical cells utilizing a nonaqueous, liquid electrolyte, an alkaline earth metal anode and a cathode capable of intercalation.
2. Description of the Prior Art
High energy density, rechargeable electrochemical cells have been recently developed having an alkali metal anode-active material, a transition metal chalcogenide cathode-active material, and a mixture of a lithium salt, such as lithium perchlorate, dissolved in an organic solvent as the electrolyte.
While a rechargeable cell is theoretically capable of charging and discharging indefinitely, in practice this is not obtained because of dendritic growths on the anode and degradation of the cathode with cycling. The electrolyte can also be a limiting factor, particularly where a nonaqueous electrolyte is utilized. Certain nonaqueous electrolytes can provide good performance with a given anode-cathode couple and be ineffective or be less effective with other anode-cathode couples, either because the electrolyte is not inert or because it degrades during cycling.
To obtain a battery system that is rechargeable at ambient temperatures, there are basically two directions that can be taken in selecting a cathode. The cathode can be a liquid so that reactions can readily take place; but when the cathode is a liquid, provision must be made to keep the cathode active material away from the anode, otherwise self-discharge will occur. The other alternative is to use a solid cathode that is essentially insoluble in the electrolyte but which will absorb and desorb the anode ion since solubility of the anode ion must occur reversibly during operation of the cell. Such a solid cathode, can be capable of intercalation of ions which are solubilized by the electrolyte. The electrolyte must also permit electroplating of solubilized ions at the anode; the plating of ions at the anode occurring during recharge of the cell and the intercalation of the cathode occurring during discharge of the cell.
The research conducted on alkali metal batteries utilizing an intercalation cathode such as titanium disulfide has shown the desirability of utilizing a cathode which is capable of intercalating the solubilized anode ion. The bulk of the literature dealing with intercalation reactions in battery development focuses on the use of the alkali metals, specifically lithium as anodes. In comparison, very little work has been done with respect to the use of alkaline earth metals, such as magnesium, for use as anodes and the use of cathodes capable of intercalation of alkaline earth metal ions.
With respect to insertion of magnesium ions into inorganic materials, there is disclosed in the literature the incorporation of magnesium into materials such as zeolites and graphite, particularly for use in the fields of catalysis and composite materials. In battery development, alkali metal ion intercalation is known to take place in the simple and complex transition metal oxides, sulfides, selenides, and tellurides. Layered transition metal disulfides have been extensively studied. Lithium is known to topochemically react with most of these disulfides, when used as cathodes, in stoichiometric ratios representing capacities of about 250 milliampere hours per gram of cathode material. Laboratory cells in the ten ampere hour range incorporating titanium disulfide as the cathode and lithium as the anode have achieved specific energy densities of 55 watt hours per pound at moderate discharge rates over more than 100 cycles at 50-80 percent discharge depth. Lithium anode cells having metal oxide cathodes have also been tested. Of particular interest as cathodes are molybdenum trioxide, magnanese dioxide, and chromium oxide (Cr.sub.3 O.sub.8) because such cathodes offer an energy density of about 60 watt hours per pound which is similar to that obtained with titanium disulfide.
Generally, intercalation chemistry is concerned with the insertion of metal guest ions into inorganic host structures. From a chemical standpoint intercalation is considered to be a reversible topotactic redox reaction by electron/ion transfer. Intercalation reactions are commonly viewed as correlating with a change in the electronic (oxidation) state of the host lattice. This oxidation change is typically nonintegral and non-stoichiometric in most compounds capable of intercalation.
With regard to the structure of the host lattice, three basic types are known. (1) A three-dimensional framework structure containing interconnected or isolated empty channels as lattice sites which share polyhedral faces. Examples of this type are the complex vanadium oxides, the trioxides of molybdenum and tungsten and zeolites. (2) Another example of a host lattice structure is a two-dimensional structure consisting of a neutral layered unit as the building block. Between the layers a van der Waals gap exists representing, to a diffusing ion, an array of neighboring vacant lattice positions. Examples of this type of structure are the layered transition metal disulfides. (3) A third type structure is a one-dimensional structure composed of chain type units separated by a van der Waals gap providing neighboring lattice sites. Examples of this type are the transition metal trisulfides. Of the three types, the layered systems (2) offer the greatest flexibility for ion insertion.
Klemann et al in U.S. Pat. No. 4,104,451 and U.S. Pat. No. 4,060,674 disclose alkali metal anode/chalcogenide cathode reversible batteries having organometallic alkali metal salts in combination with organic solvents as electrolytes. Lamellar transition metal chalcogenides, particularly the dichalcogenides are preferred. Titanium disulfide is most preferred for use as a cathode in the disclosed cells. Nonaqueous electrolytes containing alkali metal salts of boron or aluminum containing organic groups are disclosed.
In U.S. Pat. No. 4,069,372 to Voinov, cells are disclosed containing a solid mineral electrolyte capable of allowing selective migration of the anode metal in the form of cations. The electrolyte is coupled with a cathode capable of accepting electrons to form anions by cathodic reduction. Useful cathodes are disclosed as salts of transition metals such as a halide, an oxide, or a sulfide of a metal selected from iron, nickel, cobalt, chromium, copper or vanadium, i.e., ferrous chloride. The anode active material can be a metal from group Ia and IIa of the Periodic Table of the Elements. The electrolyte can be a salt of a metal of group Ia, IIa, IIb, or IIIb of the Periodic Table of the Elements.
Higashi et al in U.S. Pat. No. 4,511,642 disclose organoborate salts of alkali metals represented by the formula: ##STR2## in which R.sub.1 -R.sub.4 independently represent an alkyl group, an alkenyl group, a cycloalkyl group, an allyl group, an aryl group, a heterocyclic group, or a cyano group and M+ represents an alkali metal ion.
Malpass in U.S. Pat. No. 4,231,896 and U.S. Pat. No. 4,325,840 discloses organomagnesium complexes which are hydrocarbon soluble and useful as co-catalysts in combination with conventional Zeigler catalysts for polymerizing olefins, etc. and as a source of ether-free diorganomagnesium compounds.
In no one of these references is the electrochemical cell of the invention disclosed. In addition, there would be no suggestion for the use of an alkaline earth metal such as magnesium or calcium as an anode together with a nonaqueous organic solvent electrolyte containing an organometallic salt of an alkaline earth metal and a cathode capable of intercalation of an alkaline earth metal ion in view of the fact that the alkali metal anodes of the cells of the prior art are much more readily ionized than are alkaline earth metal anodes and therefore one skilled in the art would not expect that a cell containing an alkaline earth metal anode would provide suitable performance in comparison with a cell containing an alkali metal anode. Additionally, on recharge the cell must be capable of re-depositing the anode metal dissolved during discharge in a relatively pure state.